Fix: use absolute path with udev

Avoids the following: udevd[61613]: failed to execute '/lib/udev/bash'
'bash -c 'while [ ! -e /dev/mapper/....

Signed-off-by: Alexandre Marangone <alexandre.marangone@inktank.com>
Merge pull request #217 from alram/master

Fix: use absolute path with udev

Reviewed-by: Sage Weil <sage@inktank.com>